The present invention relates to an apparatus for focusing a reading laser on a recording medium. More particularly, it relates to focusing a laser by wavelength modulation.
Many devices are known in which focusing of an optical system is accomplished by creating a control signal from characteristics of reflected light signals wherein the control signal controls the physical movement of a component in the optical path, such as physically moving a lens or deforming a mirror. Such systems are disclosed in, for instance, U.S. Pat. Nos. 3,610,934, issued Oct. 5, 1971 to Turner; 4,060,325, issued Nov. 29, 1977, to Nobusawa; 4,264,810 issued Apr. 28, 1981 to Utagawa et al.; 4,295,741 issued Oct. 20, 1981 to Palma et al.; and 4,358,774 issued Nov. 9, 1982 to Wilkinson.
In U.S. Pat. No. 3,992,575 issued Nov. 16, 1976 to Velzel et al., an apparatus is disclosed which projects two radiation spots, one before and one behind a desired plane, and the radiation spots are reflected to sensors for generating a correction signal to correct the focus of the apparatus in a known manner. Means for correcting the focus is not further described.
It is known that variations in input parameters of various laser generating devices result in varying the wavelength of the laser output of such devices. For instance, varying the electrical current of a diode type laser generator results in a corresponding variance in the wavelength of the laser output by such device. Also, by varying the wavelength of light input to the amplifier chamber of a dye laser generator, the output frequency of the laser output by such generators will correspondingly change.